The invention relates to methods and apparatuses for coding and decoding a digitized original picture and, respectively, a digitized coded picture.
Such methods and such apparatuses are disclosed in J. De Lameillieure, R. Schäfer, MPEG-2-Bildcodierung für das digitale Fernsehen (MPEG-2 picture coding for digital television), Fernseh-und Kino-Technik, Volume 48, No. 3/1994, pp. 99–107, 1994.
In the known, standardized MPEG2 method, for the compression of digital moving picture sequences, that is to say for the coding of digitized pictures, coding information is assigned to pixels which are usually grouped into picture blocks or into picture objects.
Coding information shall hereinafter be understood to be, for example, brightness information (luminance information) and/or color information (chrominance information), which are assigned to the pixels of a digitized picture.
The coding information, which is originally present in the space domain, is, according to MPEG2, subjected to a Discrete Cosine Transform (DCT) and the DCT coefficients generated in this way are quantized and subjected to entropy coding.
A reconstruction picture is determined from the quantized DCT coefficients by an inverse quantization and an Inverse Discrete Cosine Transform (IDCT).
In this connection, a motion estimation between the original picture and a temporally preceding reconstruction picture is carried out.
For the coding, i.e. for the compression of the video data to be transmitted, a changeover is usually made between an inter coding mode and an intra coding mode.
In the context of the inter coding mode, the prediction error which has been determined as a result of the motion estimation as a difference value between the coding information of the picture block to be coded and the coding information of the picture block—determined by the motion estimation—of the temporally preceding picture is subjected to a DCT, quantized and fed to an entropy coding.
In the context of the intra coding mode, without taking account of coding information of temporally preceding pictures, the coding information of the original picture to be coded is subjected to a DCT and the resulting DCT coefficients are quantized and subjected to entropy coding.
Depending on the available data rate, in this case a coarse or a fine quantization of the DCT coefficients is chosen, i.e. set.
The coarser the quantization of the DCT coefficients is chosen to be, the more DCT coefficients are quantized to the value “zero” and the lower the data rate required for transmitting the coded original picture.
Often, a lower data rate is required for transmitting the prediction error than for transmitting the DCT coefficients—coded in accordance with the intra coding mode—of the original picture to be coded.
However, if the motion estimation is inaccurate, which, in particular, can often be observed at the boundaries of moving objects or at block boundaries between two respective picture blocks, the data rate for the prediction error may, however, also be higher than the data rate required for coding the original picture.
For this reason, a changeover between the inter coding mode and the intra coding mode is often provided in known coding methods, which changeover is intended to ensure that the signal transmitted is always the one which, overall, requires the lower data rate for its transmission.
The signal variance usually serves as a criterion for the decision as to whether a coding is to be carried out in accordance with the inter coding mode or in accordance with the intra coding mode. The said signal variance is calculated from the original picture directly, that is to say in the space domain. As an alternative, it could also be calculated by summation of the DCT coefficients determined, that is to say in the frequency domain. The signal with the lower sum of the DCT coefficients has the lower signal variance.
Furthermore, it is known that the low-frequency signal components of a video signal are usually well predicted by the motion-compensating prediction.
Accordingly, the corresponding DCT coefficients of low-frequency signal components of the respective original picture to be coded have a smaller amplitude than the DCT coefficients of the original signal. On account of estimation errors in the context of the motion estimation, however, the high-frequency signal components are often predicted only with inadequate quality, with the result that the DCT coefficients of the prediction error, which are hereinafter referred to as prediction coefficients, in this case have a higher amplitude than the DCT coefficients of the original picture.